1. Field of the Invention
The present invention relates to a position sensor and a device fabrication method using the sensor, which is preferably used to perform relative alignment (focusing) between a reticle and a wafer when projecting a pattern on the surface of a first object, such as the reticle, onto the surface of a second object, such as the water, in a projection aligner used in a lithography step among the steps of fabricating, for example, a semiconductor device such as an IC (integrated circuit) or LSI (large scale integrated circuit), imaging device such as a CCD (charge coupled device), display device such as a liquid crystal display panel, or device such as a magnetic head.
2. Description of the Related Art
In recent years, the degree of integration of semiconductor devices, such as an IC and LSI, has accelerated and the fine-patterning technique for a semiconductor wafer has advanced to correspond to the rise of the degree of integration. As the fine-patterning technique, various types of steppers have been proposed in which a circuit pattern image of a mask (reticle) is formed on a photosensitive substrate by a projection optical system (projection lens) to expose the photosensitive substrate by the step-and-repeat method.
In the case of these steppers, the whole surface of a wafer is exposed by repeating the steps of reducing the image of a circuit pattern on a reticle, projecting and transferring the pattern to a predetermined position on the wafer surface through a projection optical system having a predetermined reduction rate, moving a stage with the wafer mounted by a predetermined distance, and performing the transfer again.
In general, to perform the transfer of a fine circuit pattern by a stepper provided with a projection optical system, it is important to properly set exposure conditions such as an exposure value on the surface of a wafer and the focus position of the wafer (position in the optical axis direction of the projection optical system).
Therefore, in the case of a conventional stepper, the optimum exposure condition is determined by printing a pattern on a photosensitive substrate while changing at least either of the exposure conditions of the focus position and the exposure value (shutter time) for every shot in the test printing step (send ahead) before starting the mass production step and thereafter, developing the photosensitive substrate and measuring the line width of a linear pattern by an optical microscope or line-width measuring instrument.
For example, exposure is performing by keeping a focus value constant and changing an exposure value for every certain value in the transverse direction of arrangement in a shot area on a wafer and by keeping the exposure value constant and changing the focus value for every certain value in the longitudinal direction of shot arrangement.
Then, the line width of a resist pattern (L-and-S pattern) of line (L) and space (S) in each shot formed after development is measured by a scanning electron microscope (SEM) and thereby, the optimum focus position and the optimum exposure value of a projection lens are calculated.
FIG. 1A is a schematic view of an essential portion of a projection aligner provided with a conventional position sensor for detecting positional information (information in the optical axis direction) on the surface of a wafer. In FIG. 1A, reference numeral 2 denotes a reticle serving as a circuit original plate, 3 denotes a projection lens for reducing the size of an image of the original plate to 1/5 its original size and projecting it, 4 denotes a wafer coated with resist, and 7 denotes a stage for moving the wafer. By turning on a light source 1 and illuminating the reticle 2 by the luminous flux from the light source 1, a circuit pattern on the reticle 2 is focused on the wafer 4 and printed on the resist. When printing for one shot is completed, the stage 7 is step-driven and the next position is printed. Thus, a circuit pattern is printed on the whole surface of one wafer like a matrix. In the IC mass-production step, wafers are printed at a rate of approximately 60 wafers/hr.
A light source 101 and a luminous-flux position sensor 102 constitute a position detecting system for detecting the height (position in the optical axis direction) of the wafer 4. The sensor 102 outputs a position signal to a controller 9, which drives the stage 7 to make the wafer 4 present at the position at which the circuit pattern is focused on the reticle 4 by the projection lens 3.
FIGS. 1B, 1C and 1D explain the detection theory for detecting the height of the wafer 4. In FIG. 1B, a luminous flux 104 emitted from the light source 101 is reflected from a wafer surface 103 (wafer height 1) to detect the position of the luminous flux (e.g. peak position 102a shown in FIG. 1C) by the luminous-flux position sensor 102.
Then, when the wafer 4 moves to a surface 103a (wafer height 2), the reflected light moves up from position 102a on sensor 102 to a position 102b on sensor 102, which is also shown on the graph in FIG. 1D. In FIGS. 1C and 1D, the h-axis represents the position along the surface of sensor 102 and the i-axis represents the light intensity. FIG. 1D shows the above movement as a change of the luminous energy distribution on the luminous-flux position sensor. Height fluctuation value .delta. of the wafer 4 is proportional to fluctuation value .DELTA. of luminous flux position. Therefore, the height fluctuation value .delta. is detected by measuring the fluctuation value .DELTA.. Moreover, the inclination of the wafer 4 is calculated by measuring at least three points in a chip of the wafer 4.
A conventional projection aligner is housed in a clean chamber because the device becomes defective if dust attaches to the surface of the reticle or wafer. The clean chamber is provided with an air conditioning mechanism for keeping a constant temperature because the chamber includes an exothermic source such as a light source. The air conditioning mechanism circulates the constant-temperature air in the chamber in a laminar or turbulent flow and running the air flow from the top to the bottom of the projection aligner or horizontally running it nearby a stage and thereby, prevents the aligner from mechanically deforming and maintains the printing performance.
A mechanism for detecting the height of a wafer and its neighborhood are generally set at the bottom of a projection optical system and the laminar or turbulent flow is also supplied there by the air conditioning mechanism. But, the air flow spatially causes the refractive index of air to fluctuate. Therefore, the luminous flux 104 in FIG. 1B causes fluctuation of the optical path and its incoming position differs on the surface of the sensor 102. Thus, a problem occurs that the detection accuracy of the position on the surface of the wafer 4 decreases.
This inaccuracy in detection has recently become an important problem because the NA (numerical aperture) of an optical system increases as the line width of a circuit has decreased. As a result, the focal depth has decreased. Therefore, it is important to accurately adjust the height of a wafer to the best focus position of a projection optical system.